Preclinical prophylactic efficacy testing of Sm-p80-based vaccine in a nonhuman primate model of Schistosoma mansoni infection and immunoglobulin G and E responses to Sm-p80 in human serum samples from an area where schistosomiasis is endemic

Safety and efficacy of toll-like receptor agonists as therapeutic agents and vaccine adjuvants for infectious diseases in animals: a systematic review

Introduction

Strengthening global health security relies on adequate protection against infectious diseases through vaccination and treatment. Toll-like receptor (TLR) agonists exhibit properties that can enhance immune responses, making them potential therapeutic agents or vaccine adjuvants.

Methods

We conducted an extensive systematic review to assess the efficacy of TLR agonists as therapeutic agents or vaccine adjuvants for infectious diseases and their safety profile in animals, excluding rodents and cold-blooded animals. We collected qualitative and available quantitative data on the efficacy and safety outcomes of TLR agonists and employed descriptive analysis to summarize the outcomes.

Results

Among 653 screened studies, 51 met the inclusion criteria. In this review, 82% (42/51) of the studies used TLR agonists as adjuvants, while 18% (9/51) applied TLR agonist as therapeutic agents. The predominant TLR agonists utilized in animals against infectious diseases was CpG ODN, acting as a TLR9 agonist in mammals, and TLR21 agonists in chickens. In 90% (46/51) of the studies, TLR agonists were found effective in stimulating specific and robust humoral and cellular immune responses, thereby enhancing the efficacy of vaccines or therapeutics against infectious diseases in animals. Safety outcomes were assessed in 8% (4/51) of the studies, with one reporting adverse effects.

Discussion

Although TLR agonists are efficacious in enhancing immune responses and the protective efficacy of vaccines or therapeutic agents against infectious diseases in animals, a thorough evaluation of their safety is imperative to in-form future clinical applications in animal studies.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=323122.

Therapeutic and vaccinomic potential of moonlighting proteins for the discovery and design of drugs and vaccines against schistosomiasis

Despite significant and coordinated efforts to combat schistosomiasis, such as providing clean water, sanitation, hygiene, and snail control, these strategies still fall short, as regions previously thought to be disease-free have shown active schistosomiasis transmission. Therefore, it is necessary to implement integrated control methods, emphasizing vaccine development for sustainable control of schistosomiasis. Vaccination has significantly contributed to global healthcare and has been the most economically friendly method for avoiding pathogenic infections. Over the years, different vaccine candidates for schistosomiasis have been investigated with varying degrees of success in clinical trials with many not proceeding past the early clinical phase. Recently, proteins have been mentioned as targets for drug discovery and vaccine development, especially those with multiple functions in schistosomes. Moonlighting proteins are a class of proteins that can perform several functions besides their known functions. This multifunctional property is believed to have been expressed through evolution, where the polypeptide chain gained the ability to perform other tasks without undergoing any structural changes. Since proteins have gained more traction as drug targets, multifunctional proteins have thus become attractive for discovering and developing novel drugs since the drug can target more than one function. Moonlighting proteins are promising drug and vaccine candidates for diseases such as schistosomiasis, since they aid in disease promotion in the human host. This manuscript elucidates vital moonlighting proteins used by schistosomes to drive their life cycle and to ensure their survival in the human host, which can be used to develop anti-schistosomal therapeutics and vaccinomics.

The efficacy of cercarial antigen loaded on nanoparticles as a potential vaccine candidate in Schistosoma mansoni-infected mice

Schistosomiasis is one of the most common causes of morbidity and mortality from parasitic diseases. Mass treatment has proven to be insufficient because of repeated infection after treatment and the appearance of strains resistant to drug therapy. Hence, immunization is a new approach to control the disease and limit the pathological consequences of schistosomiasis. To evaluate the prophylactic effect of Cercarial antigen (CAP) loaded on chitosan nanoparticles (CSNPs) as a potential vaccine against Schistosoma mansoni-infected mice. 130 mice divided into 2 groups were used: Group I: Control groups (50 mice) subdivided into subgroup Ia (10 mice): Non-infected mice (normal control), subgroup Ib (20 mice): Schistosoma infected mice (infected control) and subgroup Ic (20 mice): Non-infected mice receiving NPs only. Group II: Vaccinated group (80 mice) subdivided equally into subgroup IIa (CAP): Received cercarial antigen and subgroup IIb (CAP + CSNP): Received cercarial antigen loaded on chitosan NPs then both vaccinated groups were infected with S. mansoni 3 weeks following the initial vaccination dose. CAP + CSNP and CAP groups showed significant reduction in adult worms count, hepatic egg count, hepatic granulomas number and size in comparison to the infected control group. Elevation of serum IgG and IgM levels, CD4+ and CD8+ T cell frequencies, IL-4, IL-10 and INF-γ levels was more significant in CAP + CSNP group than CAP group. CAP + CSNP is a promising new preparation of Schistosomal antigens that gave better results than immunization with CAP alone. CSNPs enhanced the immune and protective effect of CAP as validated by parasitological, histopathological and immunohistochemical studies.

Schistosoma mansoni vaccine candidates identified by unbiased phage display screening in self-cured rhesus macaques

Schistosomiasis, a challenging neglected tropical disease, affects millions of people worldwide. Developing a prophylactic vaccine against Schistosoma mansoni has been hindered by the parasite's biological complexity. In this study, we utilized the innovative phage-display immunoprecipitation followed by a sequencing approach (PhIP-Seq) to screen the immune response of 10 infected rhesus macaques during self-cure and challenge-resistant phases, identifying vaccine candidates. Our high-throughput S. mansoni synthetic DNA phage-display library encoded 99.6% of 119,747 58-mer peptides, providing comprehensive coverage of the parasite's proteome. Library screening with rhesus macaques' antibodies, from the early phase of establishment of parasite infection, identified significantly enriched epitopes of parasite extracellular proteins known to be expressed in the digestive tract, shifting towards intracellular proteins during the late phase of parasite clearance. Immunization of mice with a selected pool of PhIP-Seq-enriched phage-displayed peptides from MEG proteins, cathepsins B, and asparaginyl endopeptidase significantly reduced worm burden in a vaccination assay. These findings enhance our understanding of parasite-host immune responses and provide promising prospects for developing an effective schistosomiasis vaccine.

Experimental Validation of MHC Class I and II Peptide-Based Potential Vaccine Candidates for Human Papilloma Virus Using Sprague-Dawly Models

Human papilloma virus (HPV) causes cervical and many other cancers. Recent trend in vaccine design is shifted toward epitope-based developments that are more specific, safe, and easy to produce. In this study, we predicted eight immunogenic peptides of CD4+ and CD8+ T-lymphocytes (MHC class I and II as M1 and M2) including early proteins (E2 and E6), major (L1) and minor capsid protein (L2). Male and female Sprague Dawly rats in groups were immunized with each synthetic peptide. L1M1, L1M2, L2M1, and L2M2 induced significant immunogenic response compared to E2M1, E2M2, E6M1 and E6M2. We observed optimal titer of IgG antibodies (>1.25 g/L), interferon-γ (>64 ng/L), and granzyme-B (>40 pg/mL) compared to control at second booster dose (240 µg/500 µL). The induction of peptide-specific IgG antibodies in immunized rats indicates the T-cell dependent B-lymphocyte activation. A substantial CD4+ and CD8+ cell count was observed at 240 µg/500 µL. In male and female rats, CD8+ cell count for L1 and L2 peptide is 3000 and 3118, and CD4+ is 3369 and 3484 respectively compared to control. In conclusion, we demonstrated that L1M1, L1M2, L2M1, L2M2 are likely to contain potential epitopes for induction of immune responses supporting the feasibility of peptide-based vaccine development for HPV.

Pre-clinical development of a vaccine for human lymphatic filariasis

This study was conducted to optimize a fusion protein vaccine for translational development as a vaccine against the human tropical parasitic infection, lymphatic filariasis (LF). The vaccine candidate, His-tagged rBmHAXT was developed previously in our laboratory and was tested in various animal models including mouse, gerbils and Rhesus macaque where it exhibited significant levels of vaccine-induced protection. However, for commercial manufacturing and for regulatory approval for human use, there was a need to modify the vaccine antigen and its production and analytical release methods. Therefore, the major focus of this study was to develop a process for manufacturing an affinity tag-free rBmHAXT and evaluate its immunogenicity, potency and protective efficacy in both inbred and outbred mouse models, as well as in outbred gerbil models. Our results demonstrate that the tag-free rBmHAXT vaccine produced with a process suitable for cGMP production had protective properties equivalent to the original His-tagged rBmHAXT.

Vaccines for Human Schistosomiasis: Recent Progress, New Developments and Future Prospects

Schistosomiasis, caused by human trematode blood flukes (schistosomes), remains one of the most prevalent and serious of the neglected tropical parasitic diseases. Currently, treatment of schistosomiasis relies solely on a single drug, the anthelmintic praziquantel, and with increased usage in mass drug administration control programs for the disease, the specter of drug resistance developing is a constant threat. Vaccination is recognized as one of the most sustainable options for the control of any pathogen, but despite the discovery and reporting of numerous potentially promising schistosome vaccine antigens, to date, no schistosomiasis vaccine for human or animal deployment is available. This is despite the fact that Science ranked such an intervention as one of the top 10 vaccines that need to be urgently developed to improve public health globally. This review summarizes current progress of schistosomiasis vaccines under clinical development and advocates the urgent need for the establishment of a revolutionary and effective anti-schistosome vaccine pipeline utilizing cutting-edge technologies (including developing mRNA vaccines and exploiting CRISPR-based technologies) to provide novel insight into future vaccine discovery, design, manufacture and deployment.

Recent Advances and Methodological Considerations on Vaccine Candidates for Human Schistosomiasis

Schistosomiasis remains a neglected tropical disease of major public health concern with high levels of morbidity in various parts of the world. Although considerable efforts in implementing mass drug administration programs utilizing praziquantel have been deployed, schistosomiasis is still not contained. A vaccine may therefore be an essential part of multifaceted prevention control efforts. In the 1990s, a joint United Nations committee promoting parasite vaccines shortlisted promising candidates including for schistosomiasis discussed below. After examining the complexity of immune responses in human hosts infected with schistosomes, we review and discuss the antigen design and preclinical and clinical development of the four leading vaccine candidates: Sm-TSP-2 in Phase 1b/2b, Sm14 in Phase 2a/2b, Sm-p80 in Phase 1 preparation, and Sh28GST in Phase 3. Our assessment of currently leading vaccine candidates revealed some methodological issues that preclude a fair comparison between candidates and the rationale to advance in clinical development. These include (1) variability in animal models - in particular non-human primate studies - and predictive values of each for protection in humans; (2) lack of consensus on the assessment of parasitological and immunological parameters; (3) absence of reliable surrogate markers of protection; (4) lack of well-designed parasitological and immunological natural history studies in the context of mass drug administration with praziquantel. The controlled human infection model - while promising and unique - requires validation against efficacy outcomes in endemic settings. Further research is also needed on the impact of advanced adjuvants targeting specific parts of the innate immune system that may induce potent, protective and durable immune responses with the ultimate goal of achieving meaningful worm reduction.

Proteins as Targets in Anti-Schistosomal Drug Discovery and Vaccine Development

Proteins hardly function in isolation; they form complexes with other proteins or molecules to mediate cell signaling and control cellular processes in various organisms. Protein interactions control mechanisms that lead to normal and/or disease states. The use of competitive small molecule inhibitors to disrupt disease-relevant protein-protein interactions (PPIs) holds great promise for the development of new drugs. Schistosome invasion of the human host involves a variety of cross-species protein interactions. The pathogen expresses specific proteins that not only facilitate the breach of physical and biochemical barriers present in skin, but also evade the immune system and digestion of human hemoglobin, allowing for survival in the host for years. However, only a small number of specific protein interactions between the host and parasite have been functionally characterized; thus, in-depth understanding of the molecular mechanisms of these interactions is a key component in the development of new treatment methods. Efforts are now focused on developing a schistosomiasis vaccine, as a proposed better strategy used either alone or in combination with Praziquantel to control and eliminate this disease. This review will highlight protein interactions in schistosomes that can be targeted by specific PPI inhibitors for the design of an alternative treatment to Praziquantel.

Immunomics-Guided Antigen Discovery for Praziquantel-Induced Vaccination in Urogenital Human Schistosomiasis

Despite the enormous morbidity attributed to schistosomiasis, there is still no vaccine to combat the disease for the hundreds of millions of infected people. The anthelmintic drug, praziquantel, is the mainstay treatment option, although its molecular mechanism of action remains poorly defined. Praziquantel treatment damages the outermost surface of the parasite, the tegument, liberating surface antigens from dying worms that invoke a robust immune response which in some subjects results in immunologic resistance to reinfection. Herein we term this phenomenon Drug-Induced Vaccination (DIV). To identify the antigenic targets of DIV antibodies in urogenital schistosomiasis, we constructed a recombinant proteome array consisting of approximately 1,000 proteins informed by various secretome datasets including validated proteomes and bioinformatic predictions. Arrays were screened with sera from human subjects treated with praziquantel and shown 18 months later to be either reinfected (chronically infected subjects, CI) or resistant to reinfection (DIV). IgG responses to numerous antigens were significantly elevated in DIV compared to CI subjects, and indeed IgG responses to some antigens were completely undetectable in CI subjects but robustly recognized by DIV subjects. One antigen in particular, a cystatin cysteine protease inhibitor stood out as a unique target of DIV IgG, so recombinant cystatin was produced, and its vaccine efficacy assessed in a heterologous Schistosoma mansoni mouse challenge model. While there was no significant impact of vaccination with adjuvanted cystatin on adult worm numbers, highly significant reductions in liver egg burdens (45-55%, P<0.0001) and intestinal egg burdens (50-54%, P<0.0003) were achieved in mice vaccinated with cystatin in two independent trials. This study has revealed numerous antigens that are targets of DIV antibodies in urogenital schistosomiasis and offer promise as subunit vaccine targets for a drug-linked vaccination approach to controlling schistosomiasis.

Immunological Considerations for Schistosoma Vaccine Development: Transitioning to Endemic Settings

Despite mass drug administration programmes with praziquantel, the prevalence of schistosomiasis remains high. A vaccine is urgently needed to control transmission of this debilitating disease. As some promising schistosomiasis vaccine candidates are moving through pre-clinical and clinical testing, we review the immunological challenges that these vaccine candidates may encounter in transitioning through the clinical trial phases in endemic settings. Prior exposure of the target population to schistosomes and other infections may impact vaccine response and efficacy and therefore requires considerable attention. Schistosomes are known for their potential to induce T-reg/IL-10 mediated immune suppression in populations which are chronically infected. Moreover, endemicity of schistosomiasis is focal whereby target and trial populations may exhibit several degrees of prior exposure as well as in utero exposure which may increase heterogeneity of vaccine responses. The age dependent distribution of exposure and development of acquired immunity, and general differences in the baseline immunological profile, adds to the complexity of selecting suitable trial populations. Similarly, prior or concurrent infections with other parasitic helminths, viral and bacterial infections, may alter immunological responses. Consequently, treatment of co-infections may benefit the immunogenicity of vaccines and may be considered despite logistical challenges. On the other hand, viral infections leave a life-long immunological imprint on the human host. Screening for serostatus may be needed to facilitate interpretation of vaccine responses. Co-delivery of schistosome vaccines with PZQ is attractive from a perspective of implementation but may complicate the immunogenicity of schistosomiasis vaccines. Several studies have reported PZQ treatment to induce both transient and long-term immuno-modulatory effects as a result of tegument destruction, worm killing and subsequent exposure of worm antigens to the host immune system. These in turn may augment or antagonize vaccine immunogenicity. Understanding the complex immunological interactions between vaccine, co-infections or prior exposure is essential in early stages of clinical development to facilitate phase 3 clinical trial design and implementation policies. Besides well-designed studies in different target populations using schistosome candidate vaccines or other vaccines as models, controlled human infections could also help identify markers of immune protection in populations with different disease and immunological backgrounds.

Current status and future prospects of protein vaccine candidates against Schistosoma mansoni infection

Schistosomiasis is an acute and chronic tropical parasitic disease caused by blood dwelling worm of the genus Schistosoma. It is the most destructive disease globally and is a major cause of morbidity and mortality for developing countries. Three main species of schistosomes infect human beings from which S. mansoni is the most common and widespread. Over the last several decades, chemotherapy using praziquantel has been a commonly used strategy for the treatment and control of schistosomiasis. However, control programs focused exclusively on chemotherapy have been challenging because of the frequency and rapidity of reinfection and these programs were expensive. Thus, new schistosomiasis control strategies will be needed. Vaccination strategy would be an ideal tool for a significant and sustainable reduction in the transmission and disease burden of schistosomiasis. An effective anti schistosome vaccine would greatly contribute to decreasing schistosomiasis-associated morbidity via protective immune responses leading to reduced worm burdens and decreased egg production. Vaccine development is a long process that can take decades. There have been three candidate vaccines that have been produced by Good Manufacturing Procedure and entered human clinical trials for S. mansoni are Sm14, SmTSP-2, and Sm-p80. Other candidates that are in pre-clinical trials at various stages include paramyosin, Sm29, SmKI-1, and Sm23. Since the growth of several new technologies, including genomics, transcriptomics, microarrays, immunomic profiling, and proteomics, have helped in the identification of promising new target schistosome antigens. Therefore, this review considers the present status of protein vaccine candidates against Schistosoma mansoni and provides some insight on prospects vaccine design and discovery.

Fifteen Years of Sm-p80-Based Vaccine Trials in Nonhuman Primates: Antibodies From Vaccinated Baboons Confer Protection in vivo and in vitro From Schistosoma mansoni and Identification of Putative Correlative Markers of Protection

Recent advances in systems biology have shifted vaccine development from a largely trial-and-error approach to an approach that promote rational design through the search for immune signatures and predictive correlates of protection. These advances will doubtlessly accelerate the development of a vaccine for schistosomiasis, a neglected tropical disease that currently affects over 250 million people. For over 15 years and with contributions of over 120 people, we have endeavored to test and optimize Sm-p80-based vaccines in the non-human primate model of schistosomiasis. Using RNA-sequencing on eight different Sm-p80-based vaccine strategies, we sought to elucidate immune signatures correlated with experimental protective efficacy. Furthermore, we aimed to explore the role of antibodies through in vivo passive transfer of IgG obtained from immunized baboons and in vitro killing of schistosomula using Sm-p80-specific antibodies. We report that passive transfer of IgG from Sm-p80-immunized baboons led to significant worm burden reduction, egg reduction in liver, and reduced egg hatching percentages from tissues in mice compared to controls. In addition, we observed that sera from Sm-p80-immunized baboons were able to kill a significant percent of schistosomula and that this effect was complement-dependent. While we did not find a universal signature of immunity, the large datasets generated by this study will serve as a substantial resource for further efforts to develop vaccine or therapeutics for schistosomiasis.

Schistosomiasis vaccine development: update on human clinical trials

Schistosomiasis causes significant levels of morbidity and mortality in many geographical regions of the world. The disease is caused by infections with parasitic blood flukes known as schistosomes. The control of schistosomiasis over the last several decades has been centered on the mass drug administration (MDA) of praziquantel (PZQ), which is the only drug currently available for treatment. Despite the concerted efforts of MDA programs, the prevalence and transmission of schistosomiasis has remained largely unchecked due to the fact that PZQ is ineffective against juvenile schistosomes, does not prevent re-infection and the emergence of PZQ-resistant parasites. In addition, other measures such as the water, sanitation and hygiene programs and snail intermediate hosts control have had little to no impact. These drawbacks indicate that the current control strategies are severely inadequate at interrupting transmission and therefore, implementation of other control strategies are required. Ideally, an efficient vaccine is what is needed for long term protection thereby eliminating the current efforts of repeated mass drug administration. However, the general consensus in the field is that the integration of a viable vaccine with MDA and other control measures offer the best chance of achieving the goal of schistosomiasis elimination. This review focuses on the present status of schistosomiasis vaccine candidates in different phases of human clinical trials and provide some insight into future vaccine discovery and design.

Schistosomiasis and hookworm infection in humans: Disease burden, pathobiology and anthelmintic vaccines

Helminth diseases are the ancient scourges of humans and their damages are 'silent and insidious'. Of the helminth infections, schistosomiasis and hookworm infection have a great impact. This review covers information regarding vaccine candidates against schistosomiasis and hookworms that reached at least up to the phase-1 trial and literatures regarding other vaccine candidates have been excluded. For clinical manifestations, all available literatures were included, and for epidemiology and global burden of the diseases (GBD), literatures only within 2000-2019 were included. Literatures were searched surfing various databases including PubMED, Google Scholar, and Science Direct and overall over 150 literatures were identified. Globally ~250 million people are suffering from schistosomiasis, resulting 1430 thousand DALY (disability adjusted life year) per year. On the other hand, about 1.3 billion people are infected with hookworm (HW), and according to WHO, ~878 million school-age children (SAC) are at risk. HW is estimated to cause 65,000 deaths annually, accounts for 845 thousand DALYs as well as to cause 6-35.3% loss in productivity. Despite tremendous efforts, very few anthelmintic vaccine candidates such as Na-GST-1, Na-APR-1 and Na-ASP-2 against HW, and Sm28GST/Sh28GST, Sm-p80, Sm14 and Sm-TSP-1/SmTSP-2 against schistosomiasis reached up to the clinical trials. More efforts are needed to achieve the WHO targets taken against the maladies.

Vaccination or mass drug administration against schistosomiasis: a hypothetical cost-effectiveness modelling comparison

Background

Schistosomiasis is a neglected tropical disease, targeted by the World Health Organization for reduction in morbidity by 2020. It is caused by parasitic flukes that spread through contamination of local water sources. Traditional control focuses on mass drug administration, which kills the majority of adult worms, targeted at school-aged children. However, these drugs do not confer long-term protection and there are concerns over the emergence of drug resistance. The development of a vaccine against schistosomiasis opens the potential for control methods that could generate long-lasting population-level immunity if they are cost-effective.

Methods

Using an individual-based transmission model, matched to epidemiological data, we compared the cost-effectiveness of a range of vaccination programmes against mass drug administration, across three transmission settings. Health benefit was measured by calculating the heavy-intensity infection years averted by each intervention, while vaccine costs were assessed against robust estimates for the costs of mass drug administration obtained from data. We also calculated a critical vaccination cost, a cost beyond which vaccination might not be economically favorable, by benchmarking the cost-effectiveness of potential vaccines against the cost-effectiveness of mass drug administration, and examined the effect of different vaccine protection durations.

Results

We found that sufficiently low-priced vaccines can be more cost-effective than traditional drugs in high prevalence settings, and can lead to a greater reduction in morbidity over shorter time-scales. MDA or vaccination programmes that target the whole community generate the most health benefits, but are generally less cost-effective than those targeting children, due to lower prevalence of schistosomiasis in adults.

Conclusions

The ultimate cost-effectiveness of vaccination will be highly dependent on multiple vaccine characteristics, such as the efficacy, cost, safety and duration of protection, as well as the subset of population targeted for vaccination. However, our results indicate that if a vaccine could be developed with reasonable characteristics and for a sufficiently low cost, then vaccination programmes can be a highly cost-effective method of controlling schistosomiasis in high-transmission areas. The population-level immunity generated by vaccination will also inevitably improve the chances of interrupting transmission of the disease, which is the long-term epidemiological goal.

Molecular characterization of Schistosoma mansoni tegument annexins and comparative analysis of antibody responses following parasite infection

Schistosomes are parasitic blood flukes that infect approximately 250 million people worldwide. The disease known as schistosomiasis, is the second most significant tropical parasitic disease after malaria. Praziquantel is the only effective drug currently licensed for schistosomiasis and there are concerns about resistance to the drug. There has been much effort to develop vaccines against schistosomiasis to produce long-term protection in endemic regions. Surface-associated proteins, and in particular, those expressed in the body wall, or tegument, have been proposed as potential vaccine targets. Of these, annexins are thought to be of integral importance for the stability of this apical membrane system. Here, we present the structural and immunobiochemical characterization of four homologous annexins namely annexin B30, annexin B5a, annexin B7a and annexin B5b from S. mansoni. Bioinformatics analysis showed that there was no signal peptide predicted for any annexin in this study. Further analysis showed that each of all four annexin protein possesses a primary structure consisting of a short but variable N-terminal region and a long C-terminal core containing four homologous annexin repeats (I-IV), which contain five alpha-helices. The life cycle expression profile of each annexin was assessed using quantitative PCR. The results showed that the overall transcript levels of the each of four homologous annexins were relatively low in the egg stage, but increased gradually after the transition of cercariae (the invasive schistosome larvae) to schistosomula (the post-invasive larvae). Circular dichroism (CD) demonstrated that rAnnexin B30, rAnnexin B5a and rAnnexin 7a were folded, showing a secondary structure content rich in alpha-helices. The membrane binding affinity was enhanced when rAnnexin B30, rAnnexin B5a and rAnnexin 7a was incubated in the presence of Ca²⁺. All annexin members evaluated in this study were immunolocalized to the tegument, with immunoreactivity also occurring in cells and in muscle of adult parasites. All four recombinant annexins were immunoreactive and they were recognized by the sera of mice infected with S. mansoni. In conclusion, the overall results present the molecular characterization of annexin B30, annexin B5a, annexin B7a and annexin B5b from S. mansoni in host-parasite interactions and strongly suggest that the molecules could be useful candidates for vaccine or diagnostic development.

Sm-p80-based schistosomiasis vaccine: double-blind preclinical trial in baboons demonstrates comprehensive prophylactic and parasite transmission-blocking efficacy

Schistosomiasis is of public health importance to an estimated one billion people in 79 countries. A vaccine is urgently needed. Here, we report the results of four independent, double-blind studies of an Sm-p80-based vaccine in baboons. The vaccine exhibited potent prophylactic efficacy against transmission of Schistosoma mansoni infection and was associated with significantly less egg-induced pathology, compared with unvaccinated control animals. Specifically, the vaccine resulted in a 93.45% reduction of pathology-producing female worms and significantly resolved the major clinical manifestations of hepatic/intestinal schistosomiasis by reducing the tissue egg-load by 89.95%. A 35-fold decrease in fecal egg excretion in vaccinated animals, combined with an 81.51% reduction in hatching of eggs into the snail-infective stage (miracidia), demonstrates the parasite transmission-blocking potential of the vaccine. Substantially higher Sm-p80 expression in female worms and Sm-p80-specific antibodies in vaccinated baboons appear to play an important role in vaccine-mediated protection. Preliminary analyses of RNA sequencing revealed distinct molecular signatures of vaccine-induced effects in baboon immune effector cells. This study provides comprehensive evidence for the effectiveness of an Sm-p80-based vaccine for schistosomiasis.

Schistosoma mansoni antigen Sm-p80: prophylactic efficacy using TLR4 agonist vaccine adjuvant glucopyranosyl lipid A-Alum in murine and non-human primate models

Sm-p80, the large subunit of Schistosoma mansoni calpain, is a leading candidate for a schistosomiasis vaccine. The prophylactic and antifecundity efficacy of Sm-p80 has been tested in three animal models (mouse, hamster and baboon) using a multitude of vaccine formulations and approaches. In our continual effort to enhance the vaccine efficacy, in this study, we have utilized the adjuvant, synthetic hexa-acylated lipid A derivative, glucopyranosyl lipid A (GLA) formulated in aluminum (GLA-Alum) with recombinant Sm-p80. The rSm-p80+GLA-Alum immunization regimen provided 33.33%–53.13% reduction in worm burden in the mouse model and 38% worm burden reduction in vaccinated baboons. Robust Sm-p80-specific immunoglobulin (Ig)G, IgG1, IgG2a and IgM responses were observed in all immunized animals. The rSm-p80+GLA-Alum coadministration induced a mix of T-helper (Th) cells (Th1, Th2 and Th17) responses as determined via the release of interleukin (IL)-2, IL-4, IL-18, IL-21, IL-22 and interferon-γ.

Sm-p80-based vaccine trial in baboons: efficacy when mimicking natural conditions of chronic disease, praziquantel therapy, immunization, and Schistosoma mansoni re-encounter

Sm-p80-based vaccine efficacy for Schistosoma mansoni was evaluated in a baboon model of infection and disease. The study was designed to replicate a human vaccine implementation scenario for endemic regions in which vaccine would be administered following drug treatment of infected individuals. In our study, the Sm-p80-based vaccine reduced principal pathology producing hepatic egg burdens by 38.0% and egg load in small and large intestines by 72.2% and 49.4%, respectively, in baboons. Notably, hatching rates of eggs recovered from liver and small and large intestine of vaccinated animals were significantly reduced, by 60.4%, 48.6%, and 82.3%, respectively. Observed reduction in egg maturation/hatching rates was supported by immunofluorescence and confocal microscopy showing unique differences in Sm-p80 expression in worms of both sexes and matured eggs. Vaccinated baboons had a 64.5% reduction in urine schistosome circulating anodic antigen, a parameter that reflects worm numbers/health status in infected hosts. Preliminary analyses of RNA sequencing revealed unique genes and canonical pathways associated with establishment of chronic disease, praziquantel-mediated parasite killing, and Sm-p80-mediated protection in vaccinated baboons. Overall, our study demonstrated efficacy of the Sm-p80 vaccine and provides insight into some of the epistatic interactions associated with protection.

Prospects of developing a prophylactic vaccine against human lymphatic filariasis - evaluation of protection in non-human primates

Lymphatic filariasis (LF) affects 120 million people around the world and another 856 million people are at risk of acquiring the infection. Mass Drug Administration (MDA) spearheaded by the World Health Organization is the only current strategy to control this infection. Recent reports suggest that despite several rounds of MDA, elimination has not been achieved and there is a need for more stringent control strategies for control of LF. An effective prophylactic vaccine combined with MDA has significant potential. Initial trials using a prophylactic trivalent recombinant Brugia malayi heat shock protein 12.6, abundant larval transcript -2 and tetraspanin large extra-cellular loop (rBmHAT) vaccine developed in our laboratory conferred only 35% protection in macaques. Therefore, the focus of the present study was to improve the current vaccine formulation to obtain better protection in non-human primates. We made two modifications to the current formulation: (i) the addition of another antigen, thioredoxin peroxidase-2 (TPX-2) to make it a tetravalent vaccine (rBmHAXT) and (ii) the inclusion of an adjuvant; AL019 (alum plus glucopyranosyl lipid adjuvant-stable emulsion) that is known to promote a balanced Th1/Th2 response. A double-blinded vaccination trial was performed with 40 macaques that were divided into three treatment groups and one control group (n = 10/group). Vaccinated animals received 4 immunisations at 1 month intervals with 150 µg/ml of rBmHAT plus alum, rBmHAT plus AL019 or rBmHAXT plus AL019. Control animals received AL019 only. All vaccinated macaques developed significant (P ≤ 0.003) titers of antigen-specific IgG antibodies (1:20,000) compared with the controls. One month after the last dose, all macaques were challenged s.c. with 130-180 B. malayi L3s. Our results showed that seven out of 10 (70%) of macaques given the improved rBmHAXT vaccine did not develop the infection compared with AL019 controls, of which seven out of 10 macaques developed the infection. Titers of antigen-specific IgG1 and IgG2 antibodies were significantly (P ≤ 0.01) higher in vaccinated animals and there was an increase in the percentage of IL-4 and IFN-γ secreting antigen-responding memory T cells. These studies demonstrated that the improved formulation (rBmHAXT plus AL019) is a promising vaccine candidate against human lymphatic filariasis.

Schistosoma egg-induced liver pathology resolution by Sm-p80-based schistosomiasis vaccine in baboons

Schistosomiasis remains a serious chronic debilitating hepato-intestinal disease. Current control measures based on mass drug administration are inadequate due to sustained re-infection rates, low treatment coverage and emergence of drug resistance. Hence, there is an urgent need for a schistosomiasis vaccine for disease control. In this study, we assessed the anti-pathology efficacy of Schistosoma mansoni large subunit of calpain (Sm-p80)-based vaccine against schistosomiasis caused by infections with Schistosoma mansoni in baboons. We also evaluated the disease transmission-blocking potential of Sm-p80 vaccine. Immunisations with Sm-p80-based vaccine resulted in significant reduction of hepatic egg load in vaccinated baboons (67.7% reduction, p = 0.0032) when compared to the control animals, indicative of reduction in pathology. There was also a significant reduction in sizes of egg-induced granulomas in baboons immunised with Sm-p80 vaccine compared to their control counterparts. Egg hatching rate analysis revealed an overall 85.6% reduction (p = 0.0018) in vaccinated animals compared to the controls, highlighting the potential role of Sm-p80 vaccine in disease transmission. The findings on anti-pathology efficacy and transmission-blocking potential presented in this study have formed the basis for a large-scale double-blinded baboon experiment that is currently underway.

The self-curing phenomenon of schistosome infection in rhesus macaques: insight from in vitro studies

A reduction in the burden of schistosomiasis is potentially achievable by integrating a schistosomiasis vaccine with current control measures. Here, we determine parasite-specific in vitro responses of B, T, and NK cells from naive uninfected rhesus macaques to Schistosoma mansoni (Sm) egg (SmEA) and worm antigen (SmWA) preparations isolated from infected baboons. Pronounced B cell responses to SmEA and NK cell responses to both SmEA and SmWA were observed. High levels of IL-2 and IL-21 responses against Sm antigens were observed in T and non-T cells of lymph nodes (LNs) and gut lamina propria-derived lymphocytes (LPLs). Data analysis showed multifunctionality of LN-derived CD4⁺ , CD8⁺ , and CD4⁺ CD8⁺ double positive T cells against either SmWA or SmWA+SmEA antigen preparations. Distinct SmEA-specific multifunctional responses were observed in gut LPLs, suggesting simultaneous responses against egg antigens. These data provide insight into the immune effectors involved in schistosome responses by rhesus macaques.

Developing a mathematical model for the evaluation of the potential impact of a partially efficacious vaccine on the transmission dynamics of Schistosoma mansoni in human communities

BACKGROUND

There is currently no vaccine available to protect humans against infection with the schistosome digenean parasites, although candidate formulations for Schistosoma mansoni are under trial in animal models, including rodents and primates. Current strategies for the control of infection are based on mass drug administration (MDA) targeted at school-aged children of age 5 to 14 years. This approach is unlikely to eliminate exposure to infection except in settings with very low levels of transmission.

METHODS

A deterministic mathematical model for the transmission dynamics of the parasite is described and employed to investigate community level outcomes. The model is defined to encompass two different delivery strategies for the vaccination of the population, namely, infant (cohort) and mass vaccination. However, in this paper the focus is on vaccination delivered in a cohort immunisation programme where infants are immunised within the first year of life before acquiring infection. An analysis of the parasite's transmission dynamics following the administration of a partially protective vaccine is presented. The vaccine acts on parasite mortality, fecundity or/and establishment.

RESULTS

A vaccine with an efficacy of over 60% can interrupt transmission in low and moderate transmission settings. In higher transmission intensity areas, greater efficacy or higher infant vaccination coverage is required. Candidate vaccines that act either on parasite mortality, fecundity or establishment within the human host, can be similarly effective. In all cases, however, the duration of protection is important. The community level impact of vaccines with all modes of action, declines if vaccine protection is of a very short duration. However, durations of protection of 5-10 years or more are sufficient, with high coverage and efficacy levels, to halt transmission. The time taken to break transmission may be 18 years or more after the start of the cohort vaccination, depending on the intensity of the transmission in a defined location.

CONCLUSIONS

The analyses provide support for the proposition that even a partially efficacious vaccine could be of great value in reducing the burden of schistosome infections in endemic regions and hopefully could provide a template for the elimination of parasite transmission.

Sm-p80-based schistosomiasis vaccine mediated epistatic interactions identified potential immune signatures for vaccine efficacy in mice and baboons

Schistosomiasis is a neglected parasitic disease of major public health concern as it affects over 250 million people in developing countries. Currently there is no licensed vaccine available against schistosomiasis. The Schistosoma mansoni calpain protein, Sm-p80, is a leading vaccine candidate now ready to move to clinical trials. In order to better assess Sm-p80 vaccine immunogenicity; here we used a systems biology approach employing RNA-sequencing to identify gene signatures and epistatic interactions following Sm-p80 vaccination in mouse and baboon models that may predict vaccine efficacy. Recombinant Sm-p80 + CpG-oligodeoxynucleotide (ODN) vaccine formulation induced both cellular and humoral immunity genes with a predominant TH1 response as well as TH2 and TH17 gene signatures. Early gene responses and gene-network interactions in mice immunized with rSm-p80 + ODN appear to be initiated through TLR4 signaling. CSF genes, S100A alarmin genes and TNFRSF genes appear to be a signature of vaccine immunogenicity/efficacy as identified by their participation in gene network interactions in both mice and baboons. These gene families may provide a basis for predicting desirable outcomes for vaccines against schistosomiasis leading to a better understanding of the immune system response to vaccination.

Sm-p80-Based Schistosomiasis Vaccine: Preparation for Human Clinical Trials

Mass antiparasitic drug administration programs and other control strategies have made important contributions in reducing the global prevalence of helminths. Schistosomiasis, however, continues to spread to new geographic areas. The advent of a viable vaccine and its deployment, coupled with existing control efforts, is expected to make significant headway towards sustained schistosomiasis control. In 2016, Science ranked the schistosomiasis vaccine as one of the top 10 vaccines that needs to be urgently developed. A vaccine that is effective against geographically distinct forms of intestinal/hepatic and urinary disease is essential to make a meaningful impact in global reduction of the disease burden. In this opinion article, we focus on salient features of schistosomiasis vaccines in different phases of the clinical development pipeline and highlight the Sm-p80-based vaccine which is now being prepared for human clinical trials.

Schistosomiasis vaccines: where do we stand?

Schistosomiasis, caused mainly by S. mansoni, S. haematobium and S. japonicum, continues to be a serious tropical disease and public health problem resulting in an unacceptably high level of morbidity in countries where it is endemic. Praziquantel, the only drug currently available for treatment, is unable to kill developing schistosomes, it does not prevent re-infection and its continued extensive use may result in the future emergence of drug-resistant parasites. This scenario provides impetus for the development and deployment of anti-schistosome vaccines to be used as part of an integrated approach for the prevention, control and eventual elimination of schistosomiasis. This review considers the present status of candidate vaccines for schistosomiasis, and provides some insight on future vaccine discovery and design.

Still hope for schistosomiasis vaccine

Schistosomiasis is a parasitic disease caused by helminths belonging to the Schistosoma genus. Approximately 700 million people are at risk of infection and 200 million people are currently infected. Schistosomiasis is the most important helminth infection, and treatment relies solely on the drug praziquantel. Worries of praziquantel resistance as well as high disease burden are only some of the justifications which support the development of a vaccine against schistosomiasis. To date, only 2 schistosome vaccines have made it into clinical trials: Sh28GST (Bilhvax) and Sm14. However, there are several vaccine candidates, such as TSP-2, sm-p8, and Sm-Cathepsin B, which are generating promising results in pre-clinical studies. Schistosomiasis vaccine development has been an uphill battle, and there are still several hurdles to overcome in the future. Fortunately, the research groups involved in the research for vaccine development have not abandoned their work. Furthermore, in the last few years, schistosomiasis has garnered some additional attention on a global scale due to its significant impact on public health.

Development of a schistosomiasis vaccine

Schistosomiasis is a neglected tropical disease (NTD) of public health importance. Despite decades of implementation of mass praziquantel therapy programs and other control measures, schistosomiasis has not been contained and continues to spread to new geographic areas. A schistosomiasis vaccine could play an important role as part of a multifaceted control approach. With regards to vaccine development, many biological bottlenecks still exist: the lack of reliable surrogates of protection in humans; immune interactions in co-infections with other diseases in endemic areas; the potential risk of IgE responses to antigens in endemic populations; and paucity of appropriate vaccine efficacy studies in nonhuman primate models. Research is also needed on the role of modern adjuvants targeting specific parts of the innate immune system to tailor a potent and protective immune response for lead schistosome vaccine candidates with the long-term aim to achieve curative worm reduction. This review summarizes the current status of schistosomiasis vaccine development.

Do schistosome vaccine trials in mice have an intrinsic flaw that generates spurious protection data?

The laboratory mouse has been widely used to test the efficacy of schistosome vaccines and a long list of candidates has emerged from this work, many of them abundant internal proteins. These antigens do not have an additive effect when co-administered, or delivered as SWAP homogenate, a quarter of which comprises multiple candidates; the observed protection has an apparent ceiling of 40-50%. We contend that the low level of maturation of penetrating cercariae (~32% for Schistosoma mansoni) is a major limitation of the model since 68/100 parasites fail to mature in naïve mice due to natural causes. The pulmonary capillary bed is the obstacle encountered by schistosomula en route to the portal system. The fragility of pulmonary capillaries and their susceptibility to a cytokine-induced vascular leak syndrome have been documented. During lung transit schistosomula burst into the alveolar spaces, and possess only a limited capacity to re-enter tissues. The acquired immunity elicited by the radiation-attenuated (RA) cercarial vaccine relies on a pulmonary inflammatory response, involving cytokines such as IFNγ and TNFα, to deflect additional parasites into the alveoli. A principal difference between antigen vaccine protocols and the RA vaccine is the short interval between the last antigen boost and cercarial challenge of mice (often two weeks). Thus, after antigen vaccination, challenge parasites will reach the lungs when both activated T cells and cytokine levels are maximal in the circulation. We propose that "protection" in this situation is the result of physiological effects on the pulmonary blood vessels, increasing the proportion of parasites that enter the alveoli. This hypothesis will explain why internal antigens, which are unlikely to interact with the immune response in a living schistosomulum, plus a variety of heterologous proteins, can reduce the level of maturation in a non-antigen-specific way. These proteins are "successful" precisely because they have not been selected for immunological silence. The same arguments apply to vaccine experiments with S. japonicum in the mouse model; this schistosome species seems a more robust parasite, even harder to eliminate by acquired immune responses. We propose a number of ways in which our conclusions may be tested.

Workshop report: Schistosomiasis vaccine clinical development and product characteristics

A schistosomiasis vaccine meeting was organized to evaluate the utility of a vaccine in public health programs, to discuss clinical development paths, and to define basic product characteristics for desirable vaccines to be used in the context of schistosomiasis control and elimination programs. It was concluded that clinical evaluation of a schistosomiasis vaccine is feasible with appropriate trial design and tools. Some basic Preferred Product Characteristics (PPC) for a human schistosomiasis vaccine and for a veterinary vaccine for bovine use were also proposed.

Evaluation of Echinostoma liei worm, metacercaria and redia antigens for schistosomiasis control

While chemotherepeutic drugs, such as praziquantel, oxamniquine and metrifonate, are currently considered safe and effective drugs for schistosomiasis treatment, reinfection occurs frequently after drug treatment. Thus, a vaccine is sought to provide long-term treatment. Antigens from worm, metacercaria and redia of Echinostoma liei (E. liei) were purified using CNBr-activated Sepharose column, then used for immunization of mice prior to infection with Schistosomiasis mansoni. Worm burden, hepatic and intestinal eggs and oogram count was significantly reduced and that was reflected in normalization of liver architecture. This referred to a significant increase in the tested immunoglobulin level (IgM, IgG1 and IgG2).

Of monkeys and men: immunomic profiling of sera from humans and non-human primates resistant to schistosomiasis reveals novel potential vaccine candidates

Schistosoma haematobium affects more than 100 million people throughout Africa and is the causative agent of urogenital schistosomiasis. The parasite is strongly associated with urothelial cancer in infected individuals and as such is designated a group I carcinogen by the International Agency for Research on Cancer. Using a protein microarray containing schistosome proteins, we sought to identify antigens that were the targets of protective IgG1 immune responses in S. haematobium-exposed individuals that acquire drug-induced resistance (DIR) to schistosomiasis after praziquantel treatment. Numerous antigens with known vaccine potential were identified, including calpain (Smp80), tetraspanins, glutathione-S-transferases, and glucose transporters (SGTP1), as well as previously uncharacterized proteins. Reactive IgG1 responses were not elevated in exposed individuals who did not acquire DIR. To complement our human subjects study, we screened for antigen targets of rhesus macaques rendered resistant to S. japonicum by experimental infection followed by self-cure, and discovered a number of new and known vaccine targets, including major targets recognized by our human subjects. This study has further validated the immunomics-based approach to schistosomiasis vaccine antigen discovery and identified numerous novel potential vaccine antigens.

Multiple factors affect immunogenicity of DNA plasmid HIV vaccines in human clinical trials

Plasmid DNA vaccines have been licensed for use in domesticated animals because of their excellent immunogenicity, but none have yet been licensed for use in humans. Here we report a retrospective analysis of 1218 healthy human volunteers enrolled in 10 phase I clinical trials in which DNA plasmids encoding HIV antigens were administered. Elicited T-cell immune responses were quantified by validated intracellular cytokine staining (ICS) stimulated with HIV peptide pools. HIV-specific binding and neutralizing antibody activities were also analyzed using validated assays. Results showed that, in the absence of adjuvants and boosting with alternative vaccines, DNA vaccines elicited CD8+ and CD4+ T-cell responses in an average of 13.3% (95% CI: 9.8-17.8%) and 37.7% (95% CI: 31.9-43.8%) of vaccine recipients, respectively. Three vaccinations (vs. 2) improved the proportion of subjects with antigen-specific CD8+ responses (p=0.02), as did increased DNA dosage (p=0.007). Furthermore, female gender and participants having a lower body mass index were independently associated with higher CD4+ T-cell response rate (p=0.001 and p=0.008, respectively). These vaccines elicited minimal neutralizing and binding antibody responses. These findings of the immunogenicity of HIV DNA vaccines in humans can provide guidance for future clinical trials.

Two SmDLC antigens as potential vaccines against schistosomiasis

The Schistosoma mansoni transcriptome revealed new members of the dynein light chain family (DLC/LC8). The antigenicity and immunogenicity of these proteins, and their potential as vaccine candidates were investigated. Two DLC genes (DLC12_JI392413.1 and DLC13_JI387686.1) were cloned and the recombinant proteins produced in E. coli. The immunization of mice with the rDLCs, using alhydrogel as adjuvant, resulted in high titers of antibodies, indicated that these proteins are highly immunogenic. The anti-DLCs antibodies presented cross reactivity with both recombinant antigens and also recognized proteins from S. mansoni adult worm extracts. The DLC12 and DLC13 immunized animals were challenged by infection with cercariae and a protective profile was observed in three different assays, with a significant decreased in worm burden, of 43% and 51% respectively, when compared to the non-vaccinated group. The granulomas formation due to egg retention in the hepatic tissues was evaluated 45 days after infection. Smaller granulomas were observed in the liver of DLC immunized animals, up to 70% reduction in comparison to the granulomas size in the non-vaccinated animals. Fifty-five days after infection, the average size of the hepatic granulomas was still 25-35% smaller in the DLCs vaccinated groups. The interference of DLC immunization on the hepatic granuloma formation may reflect the lower worm burden and consequent decrease on the number of eggs retained in the liver, resulting in lower pro-inflammatory level in the tissue. The protective effect of DLCs immunization, decreasing the worm burden and delaying the rate of granuloma formation, suggests that these antigens should be further studied as potential vaccine candidates.

Immunology of human schistosomiasis

There is a wealth of immunologic studies that have been carried out in experimental and human schistosomiasis that can be classified into three main areas: immunopathogenesis, resistance to reinfection and diagnostics. It is clear that the bulk of, if not all, morbidity due to human schistosomiasis results from immune-response-based inflammation against eggs lodged in the body, either as regulated chronic inflammation or resulting in fibrotic lesions. However, the exact nature of these responses, the antigens to which they are mounted and the mechanisms of the critical regulatory responses are still being sorted out. It is also becoming apparent that protective immunity against schistosomula as they develop into adult worms develops slowly and is hastened by the dying of adult worms, either naturally or when they are killed by praziquantel. However, as with anti-egg responses, the responsible immune mechanisms and inducing antigens are not clearly established, nor are any potential regulatory responses known. Finally, a wide variety of immune markers, both cellular and humoral, can be used to demonstrate exposure to schistosomes, and immunologic measurement of schistosome antigens can be used to detect, and thus diagnose, active infections. All three areas contribute to the public health response to human schistosome infections.

Evaluation of a multivalent vaccine against lymphatic filariasis in rhesus macaque model

Lymphatic filariasis affects 120 million people worldwide and another 1.2 billion people are at risk of acquiring the infection. Chemotherapy with mass drug administration is substantially reducing the incidence of the infection. Nevertheless, an effective vaccine is needed to prevent the infection and eradicate the disease. Previously we reported that a multivalent fusion protein vaccine (rBmHAT) composed of small heat shock proteins 12.6 (HSP12.6), abundant larval transcript-2 (ALT-2) and large extracellular domain of tetraspanin (TSP LEL) could confer >95% protection against the challenge infection with Brugia malayi infective larvae (L3) in mouse and gerbil models. In this study we evaluated the immunogenicity and efficacy of rBmHAT fusion protein vaccine in a rhesus macaque model. Our results show that rBmHAT is highly immunogenic in rhesus macaques. All the vaccinated monkeys developed significant titers of antigen-specific IgG antibodies against each of the component antigens (16,000 for rBmHSP12.6), (24,000 for rBmALT-2) and (16,000 for rBmTSP-LEL). An in vitro antibody dependent cellular cytotoxicity (ADCC) assay performed using the sera samples from vaccinated monkeys showed that the anti-rBmHAT antibodies are functional with 35% killing of B. malayi L3s. Vaccinated monkeys also had antigen responding cells in the peripheral blood. Vaccine-induced protection was determined after challenging the monkeys with 500 B. malayi L3. Following challenge infection, 3 out of 5 vaccinated macaques failed to develop the infection. These three protected macaques had high titers of IgG1 antibodies and their PBMC secreted significantly high levels of IFN-γ in response to the vaccine antigens. The two vaccinated macaques that picked the infection had slightly low titers of antibodies and their PBMC secreted high levels of IL-10. Based on these findings we conclude that the rBmHAT vaccine is highly immunogenic and safe and can confer significant protection against challenge infections in rhesus macaques.

Urinary schistosomiasis

Schistosomiasis is the second most common socio-economically devastating parasitic disease after malaria, affecting about 240 million residents of developing countries. In Africa, it predominantly manifests as urogenital disease, and the main infective agent is Schistosoma hematobium. Endemicity is propagated by poor socio-economic status and environmental degradation due to rapid urbanization. Recreational swimming is a potent medium for the spread of disease in children and adolescents. Most affected individuals are asymptomatic. The male and female worms are equipped with an extraordinary capacity for immune evasion and are able to co-habit for several decades within the pelvic venous plexus. Eggs deposited in the bladder wall resist elimination by type 1 T lymphocytes. Instead, they are sustained by pro-fibrogenic encapsulation (as modulated by type 2 helper cells). Progressive bladder disease results in obstructive uropathy and predisposes to (mostly) squamous cell carcinoma. Schistosomal glomerulopathy manifests as a clinical spectrum of asymptomatic proteinuria, nephrosis and/or nephritic syndrome. Findings on renal biopsy may be influenced by co-morbidity with Salmonella bacteria, amyloidosis and hepatitis C infection. Potentially fatal Katayama fever and spinal radiculopathy may ensue in tourists visiting an endemic zone. Early detection by urine microscopy is hampered by low urinary excretion rates of the parasite eggs. Although useful in travelers with newly acquired disease, the results of the serological antibody assay may be false positive in residents of an endemic zone. Cystoscopy, however, may be invaluable. Due to its safety, effectiveness and once-daily dosing, praziquantel is the drug of choice. An integrated approach that includes mass chemotherapy, environmental health programs and public health education is the most cost-effective preventive strategy.

Why the radiation-attenuated cercarial immunization studies failed to guide the road for an effective schistosomiasis vaccine: A review

Schistosomiasis is a debilitating parasitic disease caused by platyhelminthes of the genus Schistosoma, notably Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum. Pioneer researchers used radiation-attenuated (RA) schistosome larvae to immunize laboratory rodent and non-human primate hosts. Significant and reproducible reduction in challenge worm burden varying from 30% to 90% was achieved, providing a sound proof that vaccination against this infection is feasible. Extensive histopathological, tissue mincing and incubation, autoradiographic tracking, parasitological, and immunological studies led to defining conditions and settings for achieving optimal protection and delineating the resistance underlying mechanisms. The present review aims to summarize these findings and draw the lessons that should have guided the development of an effective schistosomiasis vaccine.

Schistosome Vaccine Adjuvants in Preclinical and Clinical Research

There is currently no vaccine available for human use for any parasitic infections, including the helminth disease, schistosomiasis. Despite many researchers working towards this goal, one of the focuses has been on identifying new antigenic targets. The bar to achieve protective efficacy in humans was set at a consistent induction of 40% protection or better by the World Health Organisation (WHO), and although this is a modest goal, it is yet to be reached with the six most promising schistosomiasis vaccine candidates (Sm28GST, IrV5, Sm14, paramyosin, TPI, and Sm23). Adjuvant selection has a large impact on the effectiveness of the vaccine, and the use of adjuvants to aid in the stimulation of the immune system is a critical step and a major variable affecting vaccine development. In addition to a comprehensive understanding of the immune system, level of protection and the desired immune response required, there is also a need for a standardised and effective adjuvant formulation. This review summarises the status of adjuvants that have been or are being employed in schistosomiasis vaccine development focusing on immunisation outcomes at preclinical and clinical stages.

Human Schistosoma haematobium antifecundity immunity is dependent on transmission intensity and associated with immunoglobulin G1 to worm-derived antigens

Background

Immunity that reduces worm fecundity and, in turn, reduces morbidity is proposed for Schistosoma haematobium, a parasite of major public health importance. Mathematical models of epidemiological trends suggest that antifecundity immunity is dependent on antibody responses to adult-worm-derived antigen.

Methods

For a Malian cohort (age, 5–29 years) residing in high-transmission fishing villages or a moderate-transmission village, worm fecundity was assessed using the ratio of urinary egg excretion to levels of circulating anodic antigen, a Schistosoma-specific antigen that is steadily secreted by adult worms. Fecundity was modeled against host age, infection transmission intensity, and antibody responses specific to soluble worm antigen (SWA), tegument allergen-like 1, and 28-kDa glutathione-S-transferase.

Results

Worm fecundity declined steadily until a host age of 11 years. Among children, host age and transmission were negatively associated with worm fecundity. A significant interaction term between host age and transmission indicates that antifecundity immunity develops earlier in high-transmission areas. SWA immunoglobulin G1 (IgG1) levels explained the effect of transmission on antifecundity immunity.

Conclusion

Antifecundity immunity, which is likely to be protective against severe morbidity, develops rapidly during childhood. Antifecundity immunity is associated with SWA-IgG1, with higher infection transmission increasing this response at an earlier age, leading to earlier development of antifecundity immunity.

Longevity of Sm-p80-specific antibody responses following vaccination with Sm-p80 vaccine in mice and baboons and transplacental transfer of Sm-p80-specific antibodies in a baboon

Based on data obtained using vaccine efficacy studies in mice, hamsters, and baboons, the credentials of Sm-p80 as a first tier vaccine candidate for schistosomiasis have been well established. Sm-p80-based vaccine formulation(s) have consistently exhibited potent prophylactic efficacy in reducing adult worm burden following cercarial challenge and induce killing of established adult worms in chronic infection. This vaccine is protective against both intestinal and urinary schistosomiasis. In this study, the longevity of Sm-p80-specific antibody responses was studied in mice and in baboons. Robust antibody titers were detected in mice for up to 60 weeks following vaccination with Sm-p80 recombinant vaccine (Sm-p80 + GLA-SE). In the follow-up experiments to our published studies, Sm-p80-specific IgG was also detected in baboons 5-8 years following the initial vaccination with an Sm-p80 DNA vaccine. In one baboon, transfer of Sm-p80-specific antibody was detected in umbilical cord blood and in the baby. These long-lasting humoral immune response data coupled with the vaccine efficacy data in rodents and nonhuman primates further strengthens the case for Sm-p80 to be moved forward through development leading to human clinical trials.

An immunomics approach to schistosome antigen discovery: antibody signatures of naturally resistant and chronically infected individuals from endemic areas

Schistosomiasis is a neglected tropical disease that is responsible for almost 300,000 deaths annually. Mass drug administration (MDA) is used worldwide for the control of schistosomiasis, but chemotherapy fails to prevent reinfection with schistosomes, so MDA alone is not sufficient to eliminate the disease, and a prophylactic vaccine is required. Herein, we take advantage of recent advances in systems biology and longitudinal studies in schistosomiasis endemic areas in Brazil to pilot an immunomics approach to the discovery of schistosomiasis vaccine antigens. We selected mostly surface-derived proteins, produced them using an in vitro rapid translation system and then printed them to generate the first protein microarray for a multi-cellular pathogen. Using well-established Brazilian cohorts of putatively resistant (PR) and chronically infected (CI) individuals stratified by the intensity of their S. mansoni infection, we probed arrays for IgG subclass and IgE responses to these antigens to detect antibody signatures that were reflective of protective vs. non-protective immune responses. Moreover, probing for IgE responses allowed us to identify antigens that might induce potentially deleterious hypersensitivity responses if used as subunit vaccines in endemic populations. Using multi-dimensional cluster analysis we showed that PR individuals mounted a distinct and robust IgG1 response to a small set of newly discovered and well-characterized surface (tegument) antigens in contrast to CI individuals who mounted strong IgE and IgG4 responses to many antigens. Herein, we show the utility of a vaccinomics approach that profiles antibody responses of resistant individuals in a high-throughput multiplex approach for the identification of several potentially protective and safe schistosomiasis vaccine antigens.

Schistosomiasis is a neglected tropical disease that kills as many as 300,000 people each year. Mass drug administration is widely used to control schistosomiasis, but fails to prevent rapid reinfection in endemic areas. There is a desperate need for a prophylactic vaccine; however, very few candidates have been developed. Herein, we take advantage of recent advances in systems biology and longitudinal studies in schistosomiasis endemic areas to pilot an immunomics approach to the discovery of vaccine antigens. The emerging field of immunomics enables the determination of an “antibody signature” to a pathogen proteome for both resistant and susceptible individuals. We constructed the first protein microarray for a multi-cellular pathogen and probed it with sera from naturally resistant vs. susceptible individuals from a high transmission area in Northeastern Brazil. Using multi-dimensional cluster analysis, we showed that resistant individuals mounted a distinct and robust IgG1 antibody signature to a small set of newly discovered and well-characterized surface antigens in contrast to infected individuals. This antigen discovery strategy can lead to identification of several potentially protective and safe schistosomiasis vaccine antigens.

Simultaneous priming with DNA encoding Sm-p80 and boosting with Sm-p80 protein confers protection against challenge infection with Schistosoma mansoni in mice

Prophylactic efficacy of Sm-p80 was tested in the mouse model using DNA priming and boosting with protein approach. However, the novelty of the approach utilized in this study is that both the DNA priming and protein boosting was performed on a single day and no further vaccine inoculations were given to mice; the animals were challenged 1 month after the initial vaccine administration. Using this approach, significant reduction in worm burden (33 to 57 %) and marked decrease in egg retention in tissues (34 to 66%) was observed. Robust antibody titers and upregulation of cytokines (IL-1α/β, IL-12α, and IFN-γ) appears to correlate with the protection. This approach of administering vaccine on a single day could be greatly helpful in the field setting because it will eliminate the compliance issues that may arise with multiple boosters that may be required for optimal efficacy for some vaccines.

Use of an Sm-p80-based therapeutic vaccine to kill established adult schistosome parasites in chronically infected baboons

No vaccines are available for human use for any parasitic infections, including the helminthic disease schistosomiasis. Sm-p80, the large subunit of Schistosoma mansoni calpain, is a leading antigen candidate for a schistosomiasis vaccine. Prophylactic and antifecundity efficacies of Sm-p80 have been tested using a variety of vaccine approaches in both rodent and nonhuman primate models. However, the therapeutic efficacy of a Sm-p80-based vaccine had not been determined. In this study, we evaluated the therapeutic efficacy of Sm-p80 by using 2 different strategies and 3 Sm-p80-based vaccine formulations in baboons. Vaccine formulations were able to decrease established adult worms by 10%-36%, reduce retention of eggs in tissues by 10%-57%, and decrease egg excretion in feces by 13%-33%, compared with control formulations. Marked differences were observed in B and T cell immune correlates between vaccinated and control animals. This is the first report of killing of established adult schistosome worms by a vaccine. In addition to distinct prophylactic efficacy of Sm-p80, this study adds to the evidence that Sm-p80 is a potentially important antigen with both substantial prophylactic and therapeutic efficacies. These data reinforce that Sm-p80 should be moved forward along the path toward human clinical trials.

Cross-species protection: Schistosoma mansoni Sm-p80 vaccine confers protection against Schistosoma haematobium in hamsters and baboons

The ability of the Schistosoma mansoni antigen, Sm-p80, to provide cross-species protection against Schistosoma haematobium challenge was evaluated in hamster and baboon models. Pronounced reduction in worm burden (48%) and in tissue egg load (64%) was observed in hamsters vaccinated with recombinant Sm-p80 admixed with glucopyranosyl lipid adjuvant-stable emulsion (GLA-SE). Similarly, in baboons, the Sm-p80/GLA-SE vaccine produced a 25% reduction in S. haematobium adult worms and decreased the egg load in the urinary bladder by 64%. A 40% and 53% reduction in fecal and urine egg output, respectively, was observed in vaccinated baboons. A balanced pro-inflammatory (Th17 and Th1) and Th2 type of response was generated after vaccination and appears indicative of augmented prophylactic efficacy. These data on cross-species protection coupled with the prophylactic, therapeutic and antifecundity efficacy against the homologous parasite, S. mansoni, reinforces Sm-p80 as a promising vaccine candidate. It is currently being prepared for GMP-compliant manufacture and for further pre-clinical development leading to human clinical trials. These results solidify the expectation that the Sm-p80 vaccine will provide relief for both the intestinal and the urinary schistosomiasis and thus will be greatly beneficial in reducing the overall burden of schistosomiasis.

Complement plays a minimal role in Sm-p80-mediated protection against Schistosoma mansoni

Sm-p80, the large subunit of Schistosoma masoni calpain, is a leading antigen candidate for a schistosome vaccine. Prophylactic and antifecundity efficacy of Sm-p80 has been tested using a variety of vaccine approaches. However, the mechanism of Sm-p80-mediated killing is still unknown. In this study, potential role of complement in Sm-p80-mediated protection was studied using both in vitro (cobra venom factor inhibition) and in vivo using mice deficient in C3 (C3 -/-; B6.129S4-C3tm1Crr/J). In the absence of C3, Sm-p80-based vaccine was able to provide significant reduction in adult worm burden following challenge with schistosome cercariae in mice suggesting the effector functions of complement may be limited in this vaccine-induced protection.

Lipid core peptide targeting the cathepsin D hemoglobinase of Schistosoma mansoni as a component of a schistosomiasis vaccine

The self-adjuvanting lipid core peptide (LCP) system offers a safe alternative vaccine delivery strategy, eliminating the need for additional adjuvants such as CpG Alum. In this study, we adopted the LCP as a scaffold for an epitope located on the surface of the cathepsin D hemoglobinase (Sm-CatD) of the human blood fluke Schistosoma mansoni. Sm-CatD plays a pivotal role in digestion of the fluke's bloodmeal and has been shown to be efficacious as a subunit vaccine in a murine model of human schistosomiasis. Using molecular modeling we showed that S. mansoni cathepsin D possesses a predicted surface exposed α-helix (A₂₆₃K) that corresponds to an immunodominant helix and target of enzyme-neutralizing antibodies against Necator americanus APR-1 (Na-APR-1), the orthologous protease and vaccine antigen from blood-feeding hookworms. The A₂₆₃K epitope was engineered as two peptide variants, one of which was flanked at both termini with a coil maintaining sequence, thereby promoting the helical characteristics of the native A₂₆₃K epitope. Some of the peptides were fused to a self-adjuvanting lipid core scaffold to generate LCPs. Mice were vaccinated with unadjuvanted peptides, peptides formulated with Freund's adjuvants, or LCPs. Antibodies generated to LCPs recognized native Sm-CatD within a soluble adult schistosome extract, and almost completely abolished its enzymatic activity in vitro. Using immunohistochemistry we showed that anti-LCP antibodies bound to the native Sm-CatD protein in the esophagus and anterior regions of the gastrodermis of adult flukes. Vaccines offer an alternative control strategy in the fight against schistosomiasis, and further development of LCPs containing multiple epitopes from this and other vaccine antigens should become a research priority.

Refolding of the recombinant protein Sm29, a step toward the production of the vaccine candidate against schistosomiasis

Schistosomiasis is an important parasitic disease, with about 240 million people infected worldwide. Humans and animals can be infected, imposing an enormous social and economic burden. The only drug available for chemotherapy, praziquantel, does not control reinfections, and an efficient vaccine for prophylaxis is still missing. However, the tegumental protein Sm29 of Schistosoma mansoni was shown to be a promising antigen to compose an anti-schistosomiasis vaccine. Though, recombinant Sm29 is expressed in Escherichia coli as insoluble inclusion bodies requiring an efficient process of refolding, thus, hampering its production in large scale. We present in this work studies to refold the recombinant Sm29 using high hydrostatic pressure, a mild condition to dissociate aggregated proteins, leading to refolding on a soluble conformation. Our studies resulted in high yield of rSm29 (73%) as a stably soluble and structured protein. The refolded antigen presented protective effect against S. mansoni development in immunized mice. We concluded that the refolding process by application of high hydrostatic pressure succeeded, and the procedure can be scaled-up, allowing industrial production of Sm29.